Hub one disconnects when VPN router is connected

I have very slow internet speed around 7Mbps and my hub one kept tripping out which ended up with Plusnet sending me out another router. This stopped my disconnection issue but after a while, my speed dropped to 2.5 Mbps.

The Outreach engineer said what had happened was the DLM must have recorded me as having a fault because I'd been unplugging my old router to reboot it so had capped my internet speed at a lower speed.

He made some adjustments and I was back up to 7Mbps trouble now is when I reconnected my VPN-router the hub disconnects? (turns the blue light to orange) if I then disconnect the VPN-routers ethernet cable and reset the hub it works fine.

I didn't experience any problems on with the VPN-router at the lower speed?